Question:In the sentence of "salutem et apostolicam benedictionem", why all the words except "et" are accusative? Where are the VERB?

You must have a typo. It should be archiepiscopis and agree with venerabilibus fratribus. Suffraganeus is a suffragan, a bishop who is under the authority of an archbishop. The missing verb is understood to be "dicit" or "exoptat" or something similar. The Pope sends greetings to them (dative!), hence the accusative.

Bedware, thank you very much for you answer,I agreed you said that 'archiepiscopus' should be 'archiepiscopis', although the primary text of this Bull is 'archiepiscopus', maybe it's a transcripting erro.

So I should translate as follows:

Clement the bishop, servant of the servants of God, greetings and apostolic blessing to archbishops and their suffragan, our venerable brothers.